1. Field of the Invention
The present invention relates to a liquid crystal display device in which a semiconductor device such as a transistor formed on an insulator, in particular, a field effect transistor, and typically a MOS (metal oxide semiconductor) transistor, a thin film transistor (hereinafter, referred to as TFT) is used as a switching element in a pixel portion. The present invention also relates to a liquid crystal display device including a circuit or a driving circuit manufactured by using the semiconductor device, and to an electric appliance in which the liquid crystal display device is used for a display portion.
2. Description of the Related Art
Recently, there has been growing a use of the liquid crystal display device in a monitor of a personal computer or a display device of a television, in which liquid crystal is interposed between a pair of substrates and an electric field is applied to the pair of substrates to perform a display through a liquid crystal orientation.
Further, due to an improvement in a technique of crystallizing a semiconductor film, the liquid crystal display device is also realized in which a driving circuit is incorporated into one substrate.
Here, for the TFT of the driving circuit, high field effect mobility is required, whereas low leak current characteristics are required for the TFT used as the switching element in each pixel of the pixel portion. Thus, it is important to hold a charge (signal) and even a slight amount of leak current generated during a holding time at which the TFT is in an OFF state causes a deterioration of an image quality and a decrease in a contrast.
However, the TFT involves such a problem that when a semiconductor layer is irradiated with a light, optical excitation occurs to generate an optical leakage current. Therefore, it is important that a light-shielding film for covering the TFT is formed to block the light sufficiently to thereby prevent the light irradiation to the semiconductor layer or, assuming that a leak current is generated, it is important to secure a storage capacitor large enough to hold a signal in one frame period even in such a case.
Thus, as a structure in which a storage capacitor is formed and further a leak light can be shielded, a cell is disclosed in JP 2924506 B, in which a source electrode and a drain electrode are formed before an interlayer film is formed and a light-shielding film made of aluminum is formed on the interlayer film and subjected to anodic oxidation to form an anodized film made of Al2O3 on a top surface and a side surface of the light-shielding film, and then a transparent pixel electrode is formed thereon to complete the cell including the storage capacitor consisting of the light-shielding film, an Al2O3 film, and the transparent pixel electrode that are arranged in the stated order.
An electrostatic capacitance (hereinafter, referred to as a capacitance) of a storage capacitor element is in inverse proportion to a thickness of a capacitor insulating film (in this specification, referred to as a dielectric film sandwiched between a pair of electrodes constituted of a conductor) and in proportion to a dielectric constant of the capacitor insulating film and a surface area of the electrode. Therefore, in the structure disclosed in JP 2924506 B, since the capacitor insulating film between the light-shielding film and the transparent pixel electrode is made of the anodized film and maintained almost constant, a region where the light-shielding film and the transparent pixel electrode overlap each other can substantially function as the storage capacitor element.
However, in the liquid crystal display device, there arises a problem in that a defective display is caused by a defect in orientation of the liquid crystal. The defect in orientation of the liquid crystal occurs as follows. Steps or unevennesses on the surface of the pixel electrode affect the surface of an orientation film and involve uneven rubbing on the orientation film, and finally causes the defect in orientation of the liquid crystal and the deterioration of the image quality. In the structure disclosed in JP 2924506 B, as shown in FIG. 1A, the step of the pixel electrode exists in a portion outside a light-shielded region by the light-shielding film (a region transmissive of light) and there arises a problem in that the light accidentally passes therethrough due to the defect in orientation of the liquid crystal, or the like.
However, in the display device including a laminate structure consisting of a number of layers, it is unavoidable that the steps or unevennesses occur on the surface of the pixel electrode and affect the surface of the orientation film as well, followed by decrease in the contrast due to a light accidentally passing therethrough. Thus, a method of flattening the steps or unevennesses of the pixel electrode, particularly, a region transmissive of light which contributes to a display of the pixel has been considered.
Thus, a method is devised in which the light-shielding film is formed on an element substrate side and the step caused through the formation is flattened to form the pixel electrode. However, according to this method, when the storage capacitor element is formed between the light-shielding film and the pixel electrode, as shown in FIG. 1B, an interlayer distance is made large between the light-shielding film and the pixel electrode to thereby make a region functioning as the capacitor element narrow, with the result that it is impossible to secure the sufficient storage capacitor.
Due to a slight amount of off-leak current flowing while the TFT is in an OFF state, the decrease in the contrast and a nonuniform image quality as a panel are caused, so that it is unavoidable that the storage capacitor element is formed for complementation thereof. However, if the storage capacitor element is formed in another region, for example, as shown in FIG. 1C, if the storage capacitor element is constituted of a semiconductor layer, a gate insulating film, and a conductive layer formed on the gate insulating film with an active layer of the TFT extended, an opening ratio of the pixel becomes small, so that a display capacity is decreased in terms of brightness.